v°/EPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 277
Research and Development
EPA-600/S4-81-030 June 1981
Project Summary
Dependence of Nephelometer
Scattering Coefficients on
Relative Humidity: Evolution
of Aerosol Bursts
George W. Griffing
Observations on the temporal
dependence of the nephelometer scat-
tering coefficient on the ambient
relative humidity are presented and
discussed. The observations are repre-
sentative of the temporal dependence
of the scattering coefficient when the
weather at the Research Triangle Park
is dominated by an anticyclonic
weather system. By taking simultan-
eous nephelometer scattering coeffi-
cient observations at two different
relative humidities, it was possible to
conclude that with stable atmospheric
conditions:
• In general, the scattering coeffi-
cient increases from sundown to
sunup due to aerosol growth and
an increasing trend of the aerosol
number density;
• In general, the relatively rapid
increase and .subsequent de-
crease of the scattering coeffi-
cient during a 2 to 3 hour period
after sunup is due to a relatively
rapid aerosol growth and shrink-
age, and a relatively rapid
increase and decrease of the
aerosol number density.
The latter behavior of the scattering
coefficient was called an aerosol
burst. The relationship between an
aerosol burst, fumigation, and early
morning visibility deterioration is also
discussed.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
According to Section 169A of the
Clean Air Act Amendment of 1977
(Federal Register, 1978), the Environ-
mental Protection Agency has the
responsibility of preventing visibility
degradation of designated Class I
Federal areas. There can be various
causes for a degradation of the visibility.
One cause for visibility degradation is
an increase of the aerosol number
density which might be reduced by
control of certain anthropogenic emis-
sions. Another cause for visibility
degradation might beduetoan increase
of the relative humidity. With the possi-
ble exception of the control of the
emission of moisture from cooling
towers or similar structures, the control
of the relative humidity would not be
feasible for prevention of visibility
degradation.
For various Environmental Protection
Agency visibility related studies, a
nephelometer has been used. To
interpret the nephelometer data, an
understanding of the possible influence
of the relative humidity is essential. If
> US. GOVERNMENT PRINTING OFFICE 1M1-757-012/7135
-------
density increased. However, the scat-
tering coefficient could also have
increased because the relative humidity
increased, which could result in an
.increase in the size of the aerosols. A
decrease of the visibility would be
expected for either of these possible
reasons for an increase of the scattering
coefficient.
Since December 1978, studies have
been conducted at the Research Tri-
angle Park (RTP) on the dependence of
the nephelometer scattering coefficient
on the ambient relative humidity. In
order to extract more information from
these observations, simultaneous
nephelometer scattering coefficient
observations are taken at a different
(usually much smaller) relative humid-
ity. Except for brief periods for calibra-
tion, maintenance, and repair, the
observations have been taken
continuously.
In these studies, it has been found
that the most satisfactory humidity
sensor was the saturated salt (lilthium
chloride) humidity sensor. The cooled
mirror type humidity sensor was not
found to be reliable because of contami-
nation of the mirror by atmospheric
aerosols. At times, the mirror would get
contaminated within a few minutes. At
other times, it would be a day before the
mirror got contaminated. The saturated
salt humidity sensor was very much less
sensitive to contamination. Usually,
cleaning of the sensor was not neces-
sary before a couple of months of
operation.
The intake for atmospheric air to the
nephelometers were 50 m apart and 6
m above ground level. Special attention
was given to the problem of obtaining
relative humidities which were charac-
teristic of the nephelometer scattering
chamber. Wind observations were
taken at 6 m. Also, solar radiation data
were taken at 6 m. Both wind and solar
observations are useful in an interpre-
tation of the observations.
Conclusions
By operating one nephelometer at
ambient relative humidity and another
nephelometer at a smaller relative
humidity, it is possible to obtain an
insight into some of the temporal vari-
ations of the physiochemical param-
eters characterizing the atmospheric
aerosols. Thus, by comparing the scat-
tering coefficient profiles of the two
nephelometers it is possible to deduce
what effect that relative humidity had
on the scattering coefficient. In addition,
it is possible to gain insight into the
meteorological processes involved. The
important results of the study were
enumerated earlier.
The author of this Project Summary was George W. Gritting who is with the
Environmental Sciences Research Laboratory, EPA, Research Triangle Park
NC 27711.
K. L. Demerjian is the EPA Project Officer (see below).
The complete report, entitled "Dependence of Nephelometer Scattering Coeffi-
cients on Relative Humidity: Evolution of Aerosol Bursts," (Order No.
PB 81-198 293; Cost: $6.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
U S ENVIR PROTECTION AGENCX
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 60604
------- |